The present invention relates to the manufacture of a friction material and the friction material produced thereby and, more particularly, to the manufacture of a friction paper having improved cooling characteristics.
Friction members such as clutch plates and bands are widely used in mechanical devices such as automatic transmissions. Typically, the friction member is made from a highly heat-resistant material which is bonded to a backing member which consists of a plate of metal or other material resistant to heat deformation. The friction material must be compliant but resistant to fatigue, abrasion and heat, have desirable and sustainable frictional properties, and a long service life. If any one of these properties is not satisfied, the friction material may fail. Previously, asbestos fibers were included in friction materials. Now, however, due to health and environmental problems, asbestos is no longer being used. More recently, friction materials have been made by impregnating a paper or fiber non-woven mat materials with phenolic or other resins. These friction materials, however, do not rapidly dissipate the high heat generated.
A clutch plate for a wet clutch assembly includes a facing plate which interacts frictionally with an adjacent reaction plate to engage and disengage the clutch. The clutch plate facing and the adjacent reaction plate facing are submerged in a liquid medium which comprises a transmission fluid. The engagement of the clutch plate with the reaction plate causes a rapid and intense increase in heat. Under such adverse condition, decomposition of the transmission fluid is common and thermal deformation of the engaging parts has been observed, giving rise to transmission problems. It has been observed that the flow of transmission fluid through the friction material which accompanies the application of pressure to the friction plate as it engages the reaction member creates a cooling effect on the friction medium. Previously, the inherent porosity present in saturated cellulosic paper has been relied upon to provide the paths through which the transmission fluid flows when pressure is applied to engage the clutch plate with the reaction plate. The porosity of cellulosic paper is very random with respect to pore size, distribution and depth of the pores and the actual number of pores. Consequently, these materials provided non-uniform and inefficient cooling of the transmission fluid, which may contribute to reduced heat resistance and unsatisfactory coefficient of friction performance. To overcome these problems, various groove patterns have been employed on the friction facings of clutch plates for wet clutches to move the transmission fluid across the facing to remove heat and reduce the temperature gradient across the facing surface from the inner edge to the outer edge of the facing. For example, U.S. Pat. No. 4,045,608 to Todd; U.S. Pat. No. 5,101,953 to Payvar and U.S. Pat. No. 5,176,236 to Ghidorzi et al. disclose facing materials for clutch plates which are grooved to provide improved oil flow over the surface of the friction facing. Such grooves, however, have proved to be generally ineffective and even detrimental to the performance of the clutch plates because the grooves inherently reduce the effective surface area of the friction material. Accordingly, there is a need in the industry for an improved friction member which prevents or reduces thermal degradation of the friction member and eliminates or reduces thermal deformation of the engaging plates while retaining an effective surface area to improve the performance of the friction member.